In forming images on light-sensitive printing plates, a film original is usually superposed on a light-sensitive printing plate, then exposure of the printing plate is conducted through the film original. In this situation, the light-sensitive layer of the light-sensitive printing plate and the superposed film original must be brought into completely intimate contact with each other to thereby avoid forming a vaguely exposed image due to possible gaps therebetween for the purpose of producing a distinct image. Therefore, in general, a light-sensitive printing plate and a film original are disposed one over the other between a glass plate and a rubber sheet of a vacuum printing frame, and are brought into an intimate contact with each other by evacuating the space between the glass and the rubber sheet (vacuum-contacting method). However, with smooth-surfaced light-sensitive printing plates, evacuation of the space results in intimately contacting the smooth-surfaced printing plate with a film original only in the periphery thereof, which prevents the central area thereof from being evacuated. Thus, it requires quite a long time to intimately contact the film original with the whole surface of the light-sensitive printing plate, which reduces the efficiency of the plate-making procedure and makes the procedure economically disadvantageous.
In order to shorten the vacuum-contacting time, a light-sensitive printing plate having on its surface a matting layer comprising a micropattern of coated portions and non-coated portions present in a regular or randomly mixed manner and a process for its preparation have been developed as described in U.S. Pat. Nos. 4,216,289, 4,251,620 and 4,288,526. This technique forms a matting layer by coating, in a micropattern, a solution or dispersion of a matting material (a single substance or a composition) which does not exert detrimental physical or chemical influences on the light-sensitive layer of a light-sensitive printing plate and which can be easily removed upon development processing of the printing plate, to thereby form a micropattern of the material. In this case, the coating solvent to be used is desirably water or an aqueous liquid and not an organic solvent from the point of safety upon production of the printing plate.
However, the use of water or an aqueous liquid, which means to use a water-soluble resin as a major component of the composition, has involved the problem that, under highly humid conditions the matting layer can stain the film original, or the adhesion force of the matting layer to the light-sensitive layer may be too weak to resist against rubbing or pressure. Therefore, the matting layer tends to partly remain on the developed image portion which destroys the affinity of the image portion for ink. Accordingly, as is described in UK Patent Application GB 2081919A, a procedure has been developed which involves coating an aqueous solution or dispersion of a resin containing a specific monomer unit. However, this technique has been found to cause problems with developability or printing properties of the light-sensitive layer due to the use of a resin composition containing a resin which utilizes carboxyl groups as water-solubilizing group. More specifically, in order to make a copolymer containing a monomer unit having a weakly acidic group like a carboxyl group water-soluble, the introduction of about 80 wt% or more carboxyl group is necessary. However, such a copolymer has only weak film-forming properties and is susceptible to moisture. On the other hand, conversion of the copolymer to an alkali metal salt, ammonium salt or water-soluble amine salt serves to reduce the amount of carboxyl group. However, it makes the coated portions alkaline and, when a diazonium salt is used as a light-sensitive substance, the diazonium salt in a light-sensitive layer is decomposed with time. Accordingly, the denatured portion becomes difficult to develop.